1. Field of the Invention
The invention relates to an injection needle and an injection apparatus, more specifically to an injection needle and an injection apparatus used for injecting liquid drugs into a body.
2. Description of the Related Art
FIG. 1 and FIG. 2 show a typical injection needle used for injecting liquid drugs into a living body, particularly intradermal, subcutaneous, and muscle tissue layers.
As shown in these drawings, a drug injection needle 101 consists of a needle 102 and a base 104 that supports the needle 102. The needle 102 consists of a hollow needle of a constant outer diameter and is affixed to the base 104. A cutting edge 131 is formed at the proximate end of the needle 102. Furthermore, a female screw 143 is formed on the internal surface of the base 104.
The drug injection needle 101 is typically attached to the drug injection port formed on the nose of a drug container (not shown) containing a liquid drug. A male screw is formed on the drug injection port of the drug container, and the drug injection needle 101 is attached to the drug container when the female screw 143 of the drug injection needle 101 is screwed onto this male screw of the drug container.
The drug injection port of the drug container is sealed by a rubber stopper (not shown). The inside of the needle 102 communicates with the inside of the drug container when a cutting edge 131, which is formed on the proximate side of the needle 102, penetrates said rubber stopper as the drug injection needle 101 is attached to the drug container.
However, the needle 102 of the conventional drug injection needle 101 typically has a relatively large diameter of 0.25 mm or larger. Therefore, it causes a pain when the needle 102 punctures a living body or when a liquid drug is injected into a living body. The thought of puncturing with such a thick needle also causes fear or threat to patients.
Another problem is that it is extremely difficult to assemble a thin needle firmly to the base in the middle of the needle as shown in FIG. 2, if it is attempted to use a thinner needle in order to reduce the pains of patients.
Furthermore, such a thin needle tends to be too weak. Therefore, the needle may not be able to penetrate through the rubber stopper provided on the drug injection port of the drug container and may end up breaking when it is attempted to install the needle on the drug container.
Moreover, such a thin needle inherently has a thin inner diameter so that its liquid path resistance during injection (“injection resistance”), aggravated by its relatively long length, becomes extremely large. Thus, it has a problem that it requires a substantially large force to push out the liquid drug during injection.
More specifically, in case of the drug injection needle used by diabetes patients for intravenous self-injection of insulin solutions, even the thinnest one available on the market has an outer diameter of 0.254 mm (31 gage (G)). The “gage” is a number based on the B.W.G. (Birmingham Wire Gage).
When this needle with an outer diameter of 0.254 mm (31 G) is used, the injection resistance becomes fairly high as a result of the fact that the injection resistance of the needle increases in inverse proportion with the fourth power of the diameter of the needle. This makes it difficult for a person, who is a physically or mentally weak patient and doesn't have a sufficient force to push the plunger of the drug injection apparatus, such as a woman, child or an aged person, to inject liquid drugs subcutaneously using needles with an outer diameter of 0.254 mm (31 G). In such a case, the patient has to use thicker needles such as the one with an outer diameter of 0.30 mm (30 G).
When a needle with a 0.254 mm (31 G) outer diameter is used, the insulin solution sometimes overflows from the punctured opening of the skin or the tip of the needle. One of the reasons why it happens is suspected to be that the patient tends to pull out the needle prematurely without waiting for a specified amount of the liquid drug (specified insulin unit) to be completely administered subcutaneously because it takes too long for the liquid drug to be completely injected because of the high liquid path resistance of such a thin needle with an outer diameter of 0.254 mm (31 G). For a diabetes patient, it is important to accurately administer the specified amount of the liquid drug, which is determined by the doctor. Overflowing of the liquid drug after injection means that less than the specified amount of the liquid drug is administered and that the treatment is not sufficiently performed.
An example in another field is a dental drug injection apparatus used to inject a liquid anesthetic agent (lidocaine) into dental pulp. The outer diameter of the thinnest needle (dental needle) for the dental drug injection apparatus available on the market is 0.26 mm. There is also a substantially high injection resistance in injecting the anesthetic agent into dental pulp, etc., using a needle with an outer diameter of 0.26 mm. The main body and the plunger of the dental drug injection apparatus are made of metal and a doctor who is a healthy person operates them. Therefore, it is possible to inject the liquid anesthetic agent even with such a thin needle as the doctor can push the plunger very hard.
However, it is preferable to be able to inject with a smaller force and also to use a thinner needle in such a dental drug injection apparatus in order to reduce the pain it gives to the patient. A metallic injection apparatus is heavy. It will be able to provide a lighter and easier to handle dental drug injection apparatus if the injection resistance of the needle can be reduced so that a lighter material such as plastic can be used.
As can be seen from the above, it has been impossible to reduce the outer diameter of the needle to alleviate the patient's pain because of those problems mentioned above.